This invention relates to improvements in low-pollution external combustion engines of the type which generate power by the expansion of a nonburning gas. More particularly the invention relates to improvements in a piston-driven engine of this type for maximizing the power output thereof.
The increasing demand for low-pollution automobile engines and other power plants has indicated a strong need for replacement of the internal combustion engine. The steam engine, able to capitalize on the low emission advantages of external combustion and the simplified mechanics and drive train made possible by high starting torque and a reversible engine, is one likely successor. Other possibilities include external combustion engines utilizing freon, thiophene or other similar elastic fluids. In addition to low emissions, a further advantage of external combustion engines is that they are capable of using any heat-producing combustible fuel, as well as solar or geothermal energy sources. In any automotive engine, it would appear that pistons must be utilized rather than turbines, since turbines require very high volumes, lack low-speed torque and work best at relatively constant high speeds, thereby requiring substantial gear reduction.
Despite their low-pollution and multi-fuel advantages, the relatively low power-to-weight ratio of conventional external combustion piston engines has made them unattractive for automotive use. This disadvantage has not been overcome by efficiency-improving measures such as the development of the "uniflow" principle of exhaust porting, whereby the expansible fluid flows from the end of the cylinder to exhaust ports located near the longitudinal center of the cylinder and thus does not reverse its direction of flow during exhaust. This elimination of exhaust flow through inlet ports is important because it substantially eliminates a particular type of energy loss known to those skilled in the art as "initial condensation," thereby improving the efficiency of the external combustion engine.
I previously proposed another efficiency-improving measure in my U.S. Pat. No. 3,970,055, which provided improved gas expansion by conducting the exhaust from one side of a piston to the opposite side thereof.
However, such improvements in thermal efficiency have not improved the power-to-weight ratio of external combustion engines sufficiently to make them attractive for automotive use, despite their low-pollution and multi-fuel advantages.